1. Field of the Invention
This invention relates to a coordinate input device having the construction permitting to read a position indicated on an input plane.
2. Description of the Prior Art
There is a known coordinate input device for reading at a high precision a position indicated on a plane, as exemplified in Japanese Patent Publication No. 13074/1984. In the device, scanning signals, such as rectangular pulses, are sequentially applied to a plurality of scanning lines embedded in parallel on a tablet, induction signals conveyed by the so-called rectangular pulse scanning signals on a scanning line are detected by a coordinate indicator brought into approach, which has a excitation winding for establishing alternating magnetic field on the scanning lines. Then a rectangular pulse scanning signal involved in maximum induction signal value is searched, thus the scanning line transmitting this scanning signal being correspondent to the nearest scanning line of the coordinate indicator, and used to determine the coordinate value of it. Subsequently, the distance from the nearest scanning line to the coordinate indicator, that is, a coordinate value between scanning lines is determined from the maximum value of detected induction signal and the intensity of an arbitrary induction signal adjacent to the induction signal of the maximum value. The sum of the coordinate value of the nearest scanning line and the distance to the coordinate indicator determined as described above is computed to determine the input coordinate value of the coordinate indicator.
There however is the problem encountered in the prior art that the difference between the maximum value of a detected induction signal and the value of an arbitrary induction signal adjacent to it can be used to determine the distance from the nearest scanning line to the coordinate indicator, with a not always sufficient precision to directly compute the difference because there is a non-linear relationship between the difference and the distance. Hence the computation of the coordinate value between the scanning lines must be made on the basis of the value of the above-mentioned difference read from a ROM (Short for read only memory) on which it is stored in the form of an address signal as it were, which consequently needs previous storage of a suitable table on the ROM. Another problem involved in the prior art is that the so-called normalization by division must be made in order to avoid the influence of the variable thickness of a read media, such as paper, interposed between a tablet and an alternating magnetic field generator provided at the fore end, and variation in impedance of a scanning line and in the alternating magnetic field. Besides these conjointly brings a disadvantage of high cost.